An electric discharge machining apparatus is an apparatus for generating arc discharge between a machining electrode and a workpiece to be machined (between poles) to thereby machine the workpiece. The electric discharge machining apparatus requires an electric power source for generating discharge between the poles. When a high voltage is applied between the poles or when the distance between the poles is shortened to increase the field intensity, discharge is generated due to breakdown of insulation so that removal machining can be performed on the workpiece. When performing discharge again after the termination of the discharge and the recovery of insulation, since the distance between the poles is wide, it is necessary to apply a high voltage between the poles or to make the distance between the poles narrow to increase the field intensity.
In addition, when the discharge machining apparatus is used for machining, machining is repeated several times while changing machining conditions in accordance with a target of accuracy as to dimensions and surface roughness. A step of machining a workpiece into a target shape (first machining, which will be referred to as 1st machining) is performed first. After that, a step of increasing the shape accuracy and reducing the surface roughness in accordance with the target (shape correction machining) is performed.
In the 1st step of such machining, increase in speed is required in order to increase productivity. In the process of supplying high energy for high-speed machining, there occurs a deviation between a machining shape and a target shape. Possible reasons for this include increase in liquid spray pressure to prevent disconnection during the high-speed machining, fluctuation in wire tension in each time of machining, occurrence of strain in the workpiece due to residual stress generated during machining, etc. In this manner, in the 1st machining, there arises a problem that the shape of a machined workpiece deviates from its target shape due to occurrence of random motion of the machining electrode during machining or unexpected expansion/contraction of the workpiece. Therefore, a step of correcting the shape is required after the 1st machining.
In the shape correction machining, it is required to increase the accuracy of surface roughness while correcting the shape deviation generated in the 1st machining. However, how the shape has deviated with respect to a machining advancement direction in the 1st machining depends on the shape of a machining sample, the machining advancement direction, the machining conditions, etc. In the shape correction machining, it is required to have an ability to machine a workpiece in conformation to target dimensions even when a machining amount with which the workpiece should be corrected varies in each machining place or each direction to the machining advancement direction. When the shape cannot be corrected in conformation to the target dimensions in the shape correction machining, the distance between the poles varies depending on the machining place. Thus, it is likely that the variation in surface roughness increases.
Therefore, a control method in which an interpolar average machining voltage is monitored as means for detecting the machining state during discharge machining, and a relative moving speed is controlled to make the interpolar average machining voltage consistent with a set voltage is generally used in order to improve the faculty to correct the shape.